U.S. patent number 4,064,530 [Application Number 05/740,576] was granted by the patent office on 1977-12-20 for noise reduction system for color television.
This patent grant is currently assigned to CBS Inc.. Invention is credited to William E. Glenn, Jr., Arthur Kaiser, James Kenneth Moore.
United States Patent |
4,064,530 |
Kaiser , et al. |
December 20, 1977 |
Noise reduction system for color television
Abstract
A system for reducing noise in a color video signal which
utilizes frame store integration and includes a delay or storage
device for storing a single television frame, a summing device for
adding a fractional amplitude portion of the signal stored in the
storage device to a fractional amplitude portion of the present
video signal, and a chrominance corrector circuit for altering the
chrominance component of the stored signal so as to be in the
proper phase relationship to be summed with the chrominance
component of the present video signal. The system is operative
automatically to change the fractional amplitude portion of the
stored signal fed back to the summing device as a function of the
difference between stored and present signals thereby to change the
integration time constant of the system to accommodate for motion
between the present signal and the stored frames. The system
functions as a comb filter whose tines are very narrow when there
is little or no motion between the present and stored frames and
which are automatically widened to allow motion to be portrayed
when there is relative motion between past and present signals.
Inventors: |
Kaiser; Arthur (Trumbull,
CT), Moore; James Kenneth (Springdale, CT), Glenn, Jr.;
William E. (Fort Lauderdale, FL) |
Assignee: |
CBS Inc. (New York,
NY)
|
Family
ID: |
24977135 |
Appl.
No.: |
05/740,576 |
Filed: |
November 10, 1976 |
Current U.S.
Class: |
348/621; 348/620;
348/E9.042 |
Current CPC
Class: |
H04N
9/646 (20130101) |
Current International
Class: |
H04N
9/64 (20060101); H04N 009/535 (); H04N
005/21 () |
Field of
Search: |
;358/13,21,36,85,141,142,160,167 ;325/308,65,323 ;179/2N
;343/5VQ |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Martin; John C.
Assistant Examiner: Psitos; Aristotelis M.
Attorney, Agent or Firm: Olson; Spencer E.
Claims
We claim:
1. A noise-reducing system for reducing noise contained in
television video signals arriving on an input line, said system
comprising:
summing means having first and second input terminals for adding
first and second signals respectively applied thereto to produce a
sum signal,
means for coupling a first controllable fractional amplitude
portion of the arriving video signal to the first input terminal of
said summing means,
delay means connected to receive a sum signal from said summing
means and for delaying said sum signal for a period substantially
equal to the period of one television frame,
coupling means including means for coupling a second controllable
fractional amplitude portion of the delayed signal from said delay
means to the second input terminal of said summing means, and
means for controlling in unison said first and second fractional
amplitude portions as a function of motion between the video signal
arriving on said input line and the delayed signal and for
maintaining the sum of said first and second fractional amplitude
portions equal to unity,
whereby said summing means combines a fractional amplitude portion
of each arriving television frame with a fractional amplitude
portion of the sum of portions of preceding delayed frames to
obtain an averaged noise-reduced video signal of an amplitude
equivalent to that of the arriving video signal.
2. A noise-reducing system according to claim 1, wherein said means
for controlling said first and second fractional amplitude portions
comprises:
comparator means for comparing the arriving video signal with the
delayed signal and operative in response to detected motion varying
over a range from substantially no motion to a preselected greater
amount of motion to decrease said second fractional amplitude
portion and to correspondingly increase said first fractional
amplitude portion, and operative when the detected motion exceeds
said preselected amount to reduce said second fractional amplitude
portion to zero.
3. A noise-reducing system according to claim 1, wherein said means
for controlling said first and second amplitude portions
comprises:
comparator means for comparing the arriving video signal with the
delayed signal and for counting the number of successive frames N
that have arrived since motion exceeding a preselected amount was
last detected and for controlling said second fractional amplitude
portion according to the relationship N/N+1, and for controlling
said first amplitude portion according to the relationship 1/N+1,
and operative when the detected motion exceeds said preselected
amount to reduce said second fractional amplitude portion to
zero.
4. A noise-reducing system according to claim 2 for reducing noise
contained in arriving color video signals having luminance and
chrominance components, wherein said coupling means for coupling a
portion of the delayed video signal to the second input terminal of
said summing means further includes
chrominance correcting means for modifying the chrominance
component of the delayed video signal so as to be in proper
relationship to be added without cancellation to the chrominance
component of the arriving color video signal.
5. A noise-reducing system according to claim 3 for reducing noise
contained in arriving color video signals having luminance and
chrominance components, wherein said coupling means for coupling a
portion of the delayed video signal to the second input terminal of
said summing means further includes
chrominance correcting means for modifying the chrominance
component of the delayed video signal so as to be in the proper
relationship to be added without cancellation to the chrominance
component of the arriving color video signal.
6. A noise-reducing system according to claim 4 for reducing noise
contained in arriving NTSC color video signals having 525 lines per
frame and in which the phase of the chrominance component is
inverted from frame-to-frame, wherein
said delay means has a delay period of substantially 525H, where H
is the period of one horizontal line, and wherein said chrominance
corrector means includes
means coupled to the output terminal of said delay means for
separating the stored video signal into its constituent luminance
and chrominance components,
means for inverting the separated chrominance component, and
means for recombining the inverted separated chrominance component
with the separated luminance component.
7. A noise-reducing system according to claim 5 for reducing
noise-contained in arriving NTSC color video signals having 525
lines per frame and in which the phase of the chrominance component
is inverted from frame-to-frame, wherein
said delay means has a delay period of substantially 525H, where H
is the period of one horizontal line, and wherein said chrominance
corrector means includes
means coupled to the output terminal of said delay means for
separating the stored video signal into its constituent luminance
and chrominance components,
means for inverting the separated chrominance component, and
means for recombining the inverted separated chrominance component
with the separated luminance component.
8. A noise-reducing system according to claim 6 wherein said
arriving color video signal is digitally modulated, wherein said
delay means is a digital frame store, and wherein said chrominance
corrector means comprises digital signal processing means for
separating the stored signal into its luminance and chrominance
components, inverting the separating chrominance component and
recombining the inverted separated chrominance component with the
separated luminance component.
9. A noise-reducing system according to claim 7 wherein said
arriving color video signal is digitally modulated, wherein said
delay means is a digital frame store, and wherein said chrominance
corrector means comprises digital signal processing means for
separating the stored signal into its luminance and chrominance
components, inverting the separated chrominance component and
recombining the inverted separated chrominance component with the
separated luminance component.
10. A noise-reducing system according to claim 8, wherein said
arriving color video signal is pulse code modulated, and
wherein
said comparator means includes first means for comparing
corresponding bits of code words respectively representing the
amplitude of the arriving and the stored video signal and producing
a digital difference number representative of the measure
difference, if any, between the arriving and stored video
signals,
second means for comparing said digital difference number with a
first preselected digital reference number and for producing
coefficients determinative of said second fractional amplitude
portion the values of which are proportional to the extent of the
difference between said difference number and said first
preselected reference number, and
third means operative in response to said difference number
exceeding said first reference number for causing said second
fractional amplitude portion to go to zero.
11. A noise-reducing system according to claim 9, wherein said
arriving color video signal is pulse code modulated, and
wherein
said comparator means includes first means for comparing
corresponding bits of code words respectively representing the
amplitude of the arriving and the stored video signal, and
producing a digital difference number representative of the
measured difference, if any, between the arriving and stored video
signals,
second means for comparing said digital difference number with a
digital reference number and operative to produce a first signal at
the frame rate of the arriving video signal if the difference
number is lower than the reference number, and to produce a second
signal if the difference number exceeds the reference number,
and
digital storage means connected to receive said first and second
signals and operative to count said first signals to provide an
indication of the number of frames N that have arrived since the
difference number exceeded the reference number and to be reset to
N = 0 in response to said second signal.
Description
BACKGROUND OF THE INVENTION
This invention relates to color television systems, and is more
particularly concerned with signal-processing systems for improving
the signal-to-noise ratio of color video signals.
There is a continuing desire and frequent need in the art of
generation and transmission of television to improve the
signal-to-noise ratio of the video signal. The problem of excessive
noise in the video signal is particularly acute in the currently
evolving electronic news gathering (ENG) application of television
in which a portable camera is taken into the field, often to
locations in which the available light is marginal at best, making
it necessary to increase the video gain of the camera to obtain an
acceptable picture; this increase in video gain results in an
attendant increase in the associated noise displayed in the
picture. The problem is further compounded in ENG by the fact that
the signal from the camera is usually recorded on a small portable
recorder for subsequent playback and transmission by a microwave
link to a base station. At the base station, prior to actual
broadcast of the signal, the video signal may be subjected to two
or three levels of editing, all of these steps introducing
additional noise of different forms, so that the ultimate signal
available for airing is frequently seriously degraded. Even if the
signal delivered by the camera were free of noise, the necessary
further processing degrades the signal to a degree that the quality
of the portrayal on the home receiver is less than would be
desired. A general object of the present invention is to provide a
system for reducing noise in a color video signal thereby to
improve the signal-to-noise ratio and consequently the quality of
the displayed television picture.
The present invention takes advantage of the fact that television
signals are periodic, whereas noise is aperiodic or random. In the
NTSC system of television, the lowest repetition frequency is the
frame rate, namely, 30 frames per second, with each frame
consisting of 525 lines. For the eye to preserve continuity, any
two successive frames must be very much alike, and in fact, if no
motion is present in the picture, every frame will be a
reproduction of the one that immediately precedes it. If a signal
waveform is repetitive, signal-to-noise ratio can often be improved
by making use of the redundant information inherent in repetition.
Systems employing this technique are generally classified as signal
averagers, the principle of which is described in an article by
Charles R. Trimble entitled, "What Is Signal Averaging?" appearing
in the April 1968 issue of the HEWLETT-PACKARD JOURNAL. Although
not concerned with noise reduction in color video signals, the
paper describes in a general way the principle of noise reduction
by signal averaging.
The principle of signal averaging to achieve noise reduction has
been employed in radar applications, two examples of which are
described in a paper entitled, "Signal-To-Noise Improvement Through
Integration In A Storage Tube", Harrington and Rogers, Proceedings
of the IRE, October 1950, and in a paper entitled, "Analysis Of A
Comb Filter Using Synchronously Commutated Capacitors", LePage,
Cahn and Brown, AIEE Transactions, March 1953. Both papers deal
with a class of noise reduction utilizing integration or averaging,
but do not address the effect of motion between successive periodic
signals on the effectiveness of the noise reduction. Both papers
characterize the signal averaging system as a comb filter, many
forms of which are now used in television and elsewhere because of
their effectiveness in eliminating unwanted noise energy without
affecting the wanted periodic signal. The filters described in
these two papers are of the recursive type wherein the present
signal is added to the sum of a multiplicity of earlier versions of
substantially the same signal so as to achieve in effect, an
infinite history of the periodic signal. Neither of these papers,
however, suggest how the systems might be utilized to reduce noise
in a color video signal, and, as has been noted, do not suggest a
solution to the problem of motion.
Application of the principle of recursive filtering for noise
reduction in television signals is described in a paper by Murray
J. Stateman and Murray B. Ritterman entitled, "Theoretical
Improvement In Signal To Noise Ratio of Television Signals By
Equivalent Comb Filter Technique" published in 1954 in IRE National
Convention Record, Volume 2, Part 4. This paper describes how
redundancy and knowledge of the past signals can be used to reduce
the noise reaching the television screen. On the assumptions that
the signals are approximately periodic from frame to frame, and the
deviations of the transmitted signal from periodicity are small,
and that a serious source of deterioration in picture quality is
due to random impulse type noise, by limiting the difference
between elemental signals in successive frames to a value
consistent with the portrayal of a moderate degree of motion, the
noise pulses are attenuated before reaching the television screen.
A device suggested by the authors for restraining the incoming
television signal includes an amplitude gate in which the present
video signal is compared with a signal delayed by one frame period.
The amplitude gate passes the present video signal only if it lies
within a preset range, that is, if it is within a predetermined
range of the previously accepted signal amplitude. When the present
video signal lies outside that range, then it is modified in the
amplitude gate so that it is no further from the previous signal
than the predetermined amount. The modified output signal is then
fed to the deflection and video circuits, and also to the one frame
delay where it is stored for comparison with the next incoming
corresponding signal. The stored signal is not combined with the
present incoming signal, but, rather, is compared with the present
signal and the present signal modified in some proportion of the
difference between the stored and present signals. The amount of
stored signal compared with the present signal is always constant,
which has the effect of placing a restraint on the integration
achieved by the system, which, in turn, has the effect of
restraining the motion complexity which may be portrayed. In other
words, in the Ritterman and Stateman system a small amount of
motion is allowed to take place, the authors recognizing that if
more than that amount of motion occurs the portrayal will be
severely degraded, as by smearing of the picture. Although the
Stateman and Ritterman system will achieve an improvement in
signal-to-noise ratio in a black and white television signal, it
does not adequately solve the problem of motion and is incapable of
reducing noise in a color video signal.
U.S. Pat. No. 3,875,584 discloses a noise reduction system for a
color video signal which utilizes filtering of the nonrecursive
type in that the present frame of video is summed with one or more
preceding frames delayed by one or more frame periods, as by
storage on separate channels of a disc recorder. This system deals
(somewhat inadequately) with the problem of motion between
succeeding frames by simply reducing the number of past frames that
are integrated, and does not attempt to reduce noise that may be
present in the chrominance component of the video signal. The
incoming signal is applied to a comb filter which divides the
signal into its luminance and chrominance components, and the
luminance component of up to four successive earlier frames are
stored in a multichannel disc recorder to enable summing of the
luminance component of the present frame with the luminance
component of at least one and up to four preceding frames in order
to obtain noise reduction in the luminance component. The
noise-reduced luminance component is then recombined with the
separated and appropriately delayed chrominance component of the
present signal to obtain a reconstructed video signal for broadcast
or display. While this system has the capability of reducing noise,
it by-passes the problem of dealing with noise occurring in the
chrominance component, the effect of which is highly visible in the
television display and regarded by those versed in the broadcasting
art as at least as objectionable as that produced by noise
occurring in the luminance component. Thus, the system described in
U.S. Pat. No. 3,875,584 is incapable of reducing noise in the
chrominance component of the color video signal, it does not
satisfactorily solve the motion problem in the respect that its
effectiveness in reducing noise when there is significant motion
between successive frames, and since it utilizes nonrecursive
filtering, the system requires as many storage channels as the
manner of signals one desires to sum, an aspect which obviously
contributes to the complexity and cost of the system.
Accordingly, it is a primary object of the present invention to
provide a system for reducing noise in a color video signal which
overcomes the shortcomings of prior systems, and more particularly
to provide a system that reduces noise in both the luminance and
chrominance components of the signal while more adequately solving
the problem of motion than was achievable with prior systems.
SUMMARY OF THE INVENTION
Briefly, the system according to the invention includes a delay or
storage device for storing a single television frame, a summing
device for adding a portion of the amplitude of the stored signal
to a portion of the amplitude of the signal representing a
corresponding present frame, and a chrominance corrector circuit
for altering the phase of the chrominance component of the stored
signal such that it is in the appropriate phase to be summed with
the chrominance component of the present video signal. In a system
designed for use in the NTSC television, in which the phase of the
chrominance component is inverted from frame to frame, the
chrominance corrector consists of a chroma inverter which separates
the chrominance and luminance components of the stored signal,
inverts the chrominance component, and recombines the inverted
chrominance component with the luminance component, with the
recombined signal applied to the summing device. The portion of the
stored signal that is added to the incoming, or present signal, is
automatically changed as a function of the difference between the
stored past frames and the present frame, thus producing the effect
of changing the integration time constant of the system. More
specifically, as the difference between the stored past signal and
the present signal increases, indicating motion of the present
signal with respect to the stored past signals, the portion of the
stored signal fed back to the summer is decreased, resulting in a
faster integration time constant which, in turn, has the effect of
broadening the "tines" of the comb filter thereby to allow motion
to be portrayed while at the same time achieving significant noise
reduction. Conversely, when there is little or no motion between
the present and past signals, the portion of the stored signal fed
back to the summer is increased and the integration time constant
is slower, which has the effect of narrowing the tines of the comb
filter thereby to more effectively reduce the unwanted noise.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention will be
evident, and its construction and operation better understood, from
the following detailed description taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a block diagram of a preferred embodiment of a
noise-reducing system according to the invention;
FIG. 2 is a functional block diagram of a circuit for evaluating
motion between past and present video frames;
FIG. 3 is a curve showing signal-to-noise improvement as a function
of the fraction of the stored video signal fed back to the summer
in the system of FIG. 1;
FIG. 4 is a curve showing the effective integration time constant
as a function of the fraction of the stored video signal fed back
to the summer in the system of FIG. 1; and
FIG. 5 is a block diagram of an alternative form of noise reducing
system embodying the principles of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Although the noise reducing system of the invention is useful for
improving the signal-to-noise ratio of the color video signal in
any of the currently used color television systems, the invention
will be described as applied to the NTSC television, along with
appropriate indication of the modifications required to accommodate
to other systems, such as PAL and SECAM. Further, although the
system may be implemented in either the analog or the digital
domain, a digital implementation of the system will be
described.
Referring now to FIG. 1, the video input signal on input line 10,
which may be encoded by the pulse code modulation (PCM) technique
described in U.S. Pat. No. 3,946,432 utilizing an 8-bit code, is
applied via a variable attenuator 12 to one input terminal of an
adding or summing circuit 14. The output signal from the summer 14
is applied to a delay device 16 having a delay of 525H - .tau.; H
represents one television line interval which means that the delay
device introduces a one frame delay since there are 525 lines per
frame in the NTSC system. In a practical system the delay is
actually slightly less than one frame by the period .tau. for
reasons that will become apparent as the description proceeds. The
output of delay device 16 is applied through a chroma inverter 18
and a variable attenuator 20 to a second input terminal of the
summer 14. Attenuators 12 and 20, shown very schematically in FIG.
1, are ganged and respectively introduce a transmission constant of
(1-a) and "a". That is, a fractional portion (1-a) of the amplitude
of the incoming or "present" video signal is applied to one input
to the summer 14 and a fractional portion "a" of the amplitude of
the stored video signal from delay device 16 is applied to the
other summer input. It will be evident that if the value of "a" is
increased, for example, the proportion of the stored signal applied
to the summer increases, and the proportion of the present video
signal applied to the summer decreases. Conversely, if "a" is
decreased a larger proportion of the present signal and a smaller
proportion of the stored signal are applied to the summer.
Ignoring for the moment the presence of the chroma inverter 18
(that is, assume a direct connection from the output of delay
device 16 to attenuator 20), the described configuration is an
infinite memory system in which a fractional amplitude portion of
the sum of all previous or earlier frames are added to a fractional
amplitude portion of the incoming or present video signal, the
relative importance of a signal n frames back being determined by
the value of "a". For example, if "a" were equal to 1/2 and (1-a)
were equal to 1/2, the output signal on the line 22 would consist
of 1/2 of the present signal, 1/4 of the next earlier frame, 1/8 of
the next frame behind that, 1/16 of the next frame behind that, and
so on, with the consequence that the signal seven or eight frames
from the present signal has little significance. If there is no
motion between successive frames, the video signals representing
the successive frames will be signals identical in information
content; only the amount of noise in each will differ. When a
multiplicity of such identical signals are summed, in the manner
just described, the result is a signal identical to any one of the
summed signals and of the same magnitude as the attenuated incoming
signal by virtue of the fact that the sum of "a" and (1-a) is
always unity. However, when random noise present in the video
signal, which may vary in amount and distribution from frame to
frame, is summed, it tends to be canceled or in any case is not
reinforced as is the periodic video signal. It can be demonstrated
mathematically that the improvement in the signal-to-noise power
ratio achievable, with the described configuration is equal to
##EQU1## Thus, if the value of "a" were 1/2, the value of the
fraction would be three indicating a 4.7db signal-to-noise ration
improvement. Similarly, if "a" were greater, for example, 3/4, the
signal-to-noise improvement would be 8.45db.
It is significant to note that the recursive form of filter
provided by the system of FIG. 1 requires only one delay element
having a memory equal to 525H. It will be appreciated, however,
that because the television signal is recirculated frame after
frame and added to itself, the delay period must be extremely
precise in order for successive frames to be added properly. Unless
successive frames are in precise alignment, there will be a
degradation of the signal. Not only must the frame-to-frame
precision be satisfied, but even a higher degree of precision is
dictated in the case of a color video signal because if the delay
is not precisely 525H (less a suitable compensating delay) there
will be a phase shift of the chrominance component on successive
frames which could wipe out all of the color information in the
recirculation process. Further, because in the NTSC system of
television the phase of the chrominance component is reversed from
frame-to-frame, unless this phase reversal is accounted for in the
stored signal fed back to the summer 14, the sum of all of the
chrominance contributions in the fed back signal would be zero. An
important feature of the present invention is the provision of a
method for retaining the chrominance information and integrating it
along with the luminance information.
This function of the system is achieved by the chroma inverter 18,
the action of which has been disregarded in the description thus
far. The chroma inverter, of which both analog and digital versions
are commercially available, accepts the signal from the storage
device 16, separates the luminance component from the chrominance
component, inverts the chrominance component and then recombines
the inverted chrominance with the luminance for application to the
attenuator 20 and the summer 14. This process is indicated by block
18 by the expression (Y+C) where Y represents luminance and C
represents inverted chrominance. Analysis of NTSC video will
demonstrate that this process puts the color phase of the stored
signal, that is, the sum of the past signals, in the same phase as
the chrominance component of the present or incoming signal applied
to the other terminal of the summer, thus enabling combination of
the total incoming color video signal with the total stored color
video signal. Although the details of the chroma inverter do not
form a part of the present invention, it may employ a comb filter
to separate the luminance from the chrominance and a circuit for
inverting the chrominance component preparatory to recombination
with the separated luminance component. Again, the comb filter and
inverter may be either digital or analog, and, as has been noted,
both forms are commercially available.
It will be recognized that the chroma inverter will introduce a
certain amount of delay to the signal fed back from the delay
device 16 to the summer 14; this delay, and such other incidental
delays that may be present in the feedback loop, has been
identified in the chroma inverter block as .tau.. The important
consideration being that the total delay from the output of the
summer to the second input of the summer must be exactly 525H, the
delay of the storage device 16 must be less then 525H by the period
.tau..
Although the principle embodied in the system of FIG. 1 is
applicable to both analog and digital signal processing, from the
standpoint of achieving the necessary precision or accuracy of the
delay, the digital form of processing has been found to produce far
more satisfactory results. In a system which has been successfully
operated, the 525H delay is in the form of a digital frame store
having the capability of storing one pulse code modulated video
frame. The particular form of the frame store is unimportant, and
may be implemented with shift registers, random access memories or
any other form of addressable memory, the important consideration
being that with digital processing it is possible to achieve
essentially unlimited timing accuracy. In addition to providing the
accuracy requirements discussed above, digital processing enables
extremely accurate determination of the transmission constants "a"
and (1-a) so as to avoid drifts in the value of these constants
which would deleteriously affect the operation of the system.
Thus far there has been described how the system of FIG. 1 achieves
signal-to-noise improvement of a color video signal by recursive
filtering, the problem of motion between successive having been
referred to only incidentally. In accordance with another aspect of
the invention, the motion problem is solved by detecting motion
between stored frames and the present signal as the picture
proceeds element-by-element through the system, and in response to
the evaluation of such motion changes the value of the transmission
factor "a" (and consequently (1-a)) so as to alter the contribution
of the stored past signals to the noise-reduced video output
signal. If a picture element from the same scene object in the
stored past signals is sufficiently different in amplitude from the
same element in the present video signals, the past history of that
picture element is ignored and only the present signal is
transmitted to the output terminal. Although there would be no
signal-to-noise improvement for that particular picture element, it
should be noted that for the most part motion is observed only on
the borders and in the fine detail of objects, and not on the broad
areas of objects; that is, it is the interface between an object in
a scene and its background that makes motion detectable in the
displayed television picture. With this in mind, the system is
operable in response to detected motion to alter "a" and (1-a) in
such a way as to accommodate motion, in the limit allowing "a" to
go to zero.
This is accomplished in the digital system shown in the functional
diagram of FIG. 2 which, in essence, performs the function of the
summer 14 and the attenuators 12 and 20 schematically illustrated
in FIG. 1. The input signal, namely, the "present" video signal, is
received on line 10 as an 8-bit PCM encoded signal and applied to a
suitable register, schematically shown at 30, which receives the
individual bits of each word, the elements of which being labelled
"0 " for the least significant bit and "7" for the most significant
bit. The stored video from the chroma inverter 18 (also 8-bit PCM
encoded) is applied to a similar "register" 32, the elements of
which are also labelled from "0" to "7" to represent the least and
most significant bits, respectively. In order to detect motion, the
total past value of the signal (i.e., the stored video) is compared
element by element (or byte by byte in 93 nanosecond slots) with
the present video signal. That is, the 8-bit word (byte) that
represents the amplitude of the total past signal in storage is
compared bit-by-bit with the 8-bit word that represents the
amplitude of the present video signal. In the illustrated
embodiment, the least significant bit is discarded in both cases,
and because it was recognized that wide differences could not be
tolerated, two levels of comparison are employed. More
specifically, bits 1, 2, 3 and 4 of the words representing the
stored video and the present video are applied to the - and + input
terminals 34 and 36, respectively, of a difference amplifier 38,
the output from which is a 4-bit word representative of the
difference, if any, between the stored video and the present
signal. The 4-bit word from the difference amplifier is applied to
an integration comparator 40 which compares it to a 4-bit
integration reference number, schematically illustrated at 42, of
preset value much greater than zero. The output from the
integration comparator, a 3-bit word, is applied to a coefficient
decoder 44 which is operative to determine the value of the "a"
coefficient, which, as has been noted earlier, determines the
fractional amplitude proportion of the stored video signal to be
added to the present video signal. If the result of the comparison
in the integration comparator 40 is zero, meaning that the 4-bit
number from difference amplifier 38 is equal to the integration
reference number, this signifies that there is a predetermined
difference between the stored video and the present video signals
at this level of comparison. A zero difference at the output of
integration comparator 40 causes the coefficient decoder 44 to
produce a coefficient "a" of a value such as to produce a small
amount of feedback of the stored video signal. In the present
embodiment, which is to be understood as illustrative only, the
coefficient decoder 44 is operative, depending on the difference
number produced by the integration comparator, to produce one of
three values of "a", namely, one-fourth, one-half, or
three-fourths; in the case of zero difference at the output of
integration comparator 40, indicative of the greatest tolerable
motion, the coefficient "a" has a value of one-fourth. When the
difference at the output of comparator 40 is greater than zero,
indicating less motion between the stored and present video than
previously, the decoder produces a coefficient having a value of
one-half, and when still less motion is detected, a coefficient
having a value of three-fourths is produced. The coefficient "a",
in the form of a digital word, is applied to a summing device,
diagrammatically shown at 46, which includes two elements 48 and 50
labelled "a" and (1 - a), respectively, to signify the relative
fractional amplitude proportions of the stored video and present
video, respectively, that are summed. The stored video is applied
to the element 48, the present video is applied to element 50, and
the sum of their respective fractional amplitude portions is
applied over line 52 as one input to a multiplexer 54 which is
operative to transmit a noise-reduced video signal to the output
line 22 in situations when the value of coefficient "a" is not
zero.
To take care of the possibility of there being no difference in
bits 1 - 4 of the stored and present video signals yet, a large
difference actually existing between the stored and present signals
which shows up in the more significant bits, a second comparison is
made between bits 5, 6 and 7 of the stored video and the
corresponding bits of the present video. To this end, the three
most significant bits of the stored and present video are applied
to the - and + input terminals 60 and 62, respectively, of a second
difference amplifier 64, the 3-bit word output of which is applied
to an OR circuit 66. When the difference amplifier 64 detects a
difference between the three most significant bits of the stored
and present video, indicating that there is motion between them in
excess of the allowable threshold established by the integration
reference 42, the OR circuit 66 applies a signal to one input of a
second OR circuit 68 which, in turn, applies a signal to
multiplexer 54 which effectively causes the coefficient "a" to go
to zero. That is, if the motion exceeds a preset amount, none of
the stored video is added to the present signal; instead, the
present signal only is transmitted to the output line 22.
Although not absolutely necessary for the operation of the system,
in order to obtain independence of action another circuit is
provided for making the value of coefficient "a" go to zero under
certain conditions. More particularly, the 4-bit word at the output
of difference amplifier 38 is also applied to a bypass comparator
70 in which it is compared to a bypass threshold number
schematically shown at 72, which is a 4-bit word of a value
somewhat greater than the value of the integration reference number
42. When the difference in the output of difference amplifier 38
exceeds the bypass threshold number, bypass comparator 70 produces
an output signal which is applied to a second input of OR circuit
68, which produces an output signal which is applied to multiplexer
54 to cause the value of coefficient "a" to go to zero. Thus, the
coefficient "a" will go to zero, causing only the present video
signal to be coupled to the output line 22, when (1) there is any
difference between the three most significant bits of the stored
and present video signals, or (2) the difference between four less
significant bits of the stored and present video signals exceeds a
preselected bypass threshold number.
Summarizing the operation of the system of FIG. 2, when comparison
of the stored and present video indicates less than a predetermined
amount of motion, the system automatically operates to change the
value of coefficient "a" in response to the amount of motion
detected. In this situation, the summer 46 combines the present
video in the proportion (1 - a) with the stored video in the
proportion "a" , and transmits the sum signal to the "not equal to
zero" input of multiplexer 54 which, in turn, couples the sum
signal to the output line 22. However, when the system detects
motion between the stored and present video in excess of a
predetermined amount, the coefficient "a" is reduced to zero, in
which case only the present video signal is coupled to the output
line 22. It is important to note that when the value of "a" is
caused to go to zero, the entire past history of that picture
element in which excessive motion was detected is lost and only the
present video representing that element is used; thereafter, the
frame store has to build up a new past history for that particular
element.
The described system places no restraints on the allowable motion
to be portrayed in the displayed picture. Applicants have
recognized that allowing those picture elements which have motion
in excess of a predetermined amount to go through without noise
reduction poses no serious handicap, for two reasons. First, they
are individual picture elements, not broad reas in a scene, and
being individual elements it is difficult to perceive noise in
them. Stated another way, where there is "busyness" of signal,
noise is not as apparent; on the other hand, when the signal is
relatively flat or uniform, as in broad areas of a scene, noise is
very apparent. The present system is operative to integrate when
there is uniformity of signal, thus reducing noise, but doesn't
integrate when there is movement, which normally occurs at edges
between an object and its background, but again the noise is not as
visible in this part of the display. Secondly, when an object is in
motion the eye finds it difficult to concentrate on an edge of the
object, so the fact that the object is in motion also precludes
visible detection of noise associated with the signal. In effect,
then, the system provides the best of both worlds; broad uniform
areas of objects in motion are integrated because, with the
exception of their boundaries, they are seemingly not in motion,
whereas the boundaries are not integrated (if there is a sufficient
amount of motion). Thus, the system gives the advantages of
integration without inhibiting the motion in the displayed
picture.
In effect, the described system is a variable comb filter, picture
element by picture element, having very narrow tines for some
picture elements and wider tines for other picture elements,
depending upon the amount of motion present in a particular picture
element between successive frames. Consequently, the
signal-to-noise improvement varies in accordance with motion from
frame to frame, there being no noise reduction when the motion
exceeds a predetermined amount and the noise reduction being
maximum when there is little or no motion. This property of the
system is graphically illustrated in FIG. 3, which is a curve
showing signal-to-noise improvement for different values of "a"; it
will be seen that the signal-to-noise improvement goes from zero
when "a" is zero (i.e., when only present video is coupled to the
output line) to approximately 13db when "a" has a value of the
order of 0.9, and theoretically becomes infinite when "a" is equal
to one (the case where only the stored signal is coupled to the
output of the system).
The curve in FIG. 4 shows the effective integration time constant
of the system of FIG. 1 as a function of the transmission constant
"a", the ordinate on the left showing the time constant in terms of
the number of television frames, each of which, of course, lasts
for a 30th of a second. The ordinate on the right provides an
absolute measure of the integration time constant in seconds. For
small values of "a", indicative of significant motion between the
stored and present video, the time constant is relatively short,
causing the tines of the comb filter to be widened, and at the
larger values of "a", a larger number of successive frames are
integrated, which has the effect of narrowing the tines of the comb
filter with an attendant increase in the noise reduction.
Although the system has been described for use in the NTSC system
of television, it is also useful, with suitable modification of the
circuit for coupling the stored video back to the summer, in other
known systems. For the PAL system, for example, the chroma inverter
18 would be replaced by a chrominance corrector circuit that would
alter the phase shift angle of the chrominance component of the
stored signal so as to be in the proper phase relationship to be
added to the chrominance component of the present video signal.
Similarly, it will be evident to those knowledgeable of the SECAM
system, in which the chrominance component is frequency modulated,
as to what frame-to-frame adjustment to the chrominance component
would have to be made to allow it to be appropriately added to the
chrominance component of the present video. In general, for both
PAL and SECAM, the chroma correcting circuit would require a filter
for separating the chrominance from the luminance, means for
applying the appropriate correction to the chrominance component,
and means for recombining the corrected chrominance component with
the separated luminance prior to application to the summer.
Further, although in the system of FIG. 2 values of "a" of
one-fourth, one-half, and three-fourths have been used, it is to be
understood that these are only by way of example, and theoretically
there is no limit, particularly in a digital system, on the total
number of different values "a" that may be made available. The
number is limited only by practical considerations such as the
amount of circuit complexity that one may wish to incorporate in
return for a desired degree of signal-to-noise improvement.
It is also to be understood that although the system of FIG. 1 has
been described in the context of its capability to reduce noise in
a color video signal, the principle of changing the proportions of
the stored and present video as a function of motion between past
and present signals is also applicable to monochrome television.
For a black and white application, the chroma correction circuit
simply would be omitted.
DESCRIPTION OF ALTERNATIVE EMBODIMENT
FIG. 5 is a functional block diagram of an alternative system for
improving the signal-to-noise ratio of a color video signal, and
will be described in connection with its use in the NTSC system of
television and implemented in the digital domain. As was the case
of the system of FIG. 1, the system, with suitable modification, is
useful in other known systems of television, and may also be
implemented with analog components.
The present video signal on an input line 10, PCM-encoded using an
8-bit word, is applied to a multiplying circuit 82 which is
operative to perform the function of multiplying the amplitude of
the present video signal by the factor 1/N+1 and to apply the
resultant signal as one input to a summing circuit 84. The
significance of the term "N" will become evident as the description
proceeds. The output of adder 84 is applied to a one-frame delay
device 86 which has a delay of 525H, less such delays as may be
introduced by the following circuit elements involved in coupling
the signal stored in the delay device 86 to a second input of the
summing circuit 84. The signal stored in delay device 86, which, as
in the system of FIG. 1, may be a commercially available digital
frame store, is applied to a chroma inverter 88, which may be of
the form of and providing the same function as the chroma inverter
18 in the system of FIG. 1, and the recombined signal therefrom is
applied to a second multiplying circuit 90 which is operative to
multiply the delayed or stored signal by the factor N/N+1. The
resultant signal at the output of multiplier 90 is applied as a
second input to the summing circuit 84. It is seen from the
description thus far that a fractional amplitude portion of the
present video signal is added to a different fractional portion of
the chroma-corrected stored signal, with the sum of the two
transmission coefficients being equal to unity. A noise-reduced
video signal is derived on output line 92.
In this embodiment, the ratio between the amplitude portions of the
present and stored video that are added together is determined by
how long it has been (that is, how many frames have passed) since
motion was detected in a picture element. To this end, a tag
register 92, which may be an extension of the frame store utilized
as the delay device 86, is provided for counting the number of
frames that have gone by since motion was last detected in a given
picture element. The tag store has a capacity to store a word of
length "N" for each picture element, where N equals the number of
frames since motion was detected for each pixel. Since it is not
necessary to integrate more than seven or eight past frames, the
value of "N" can be readily stored in a 3-bit register. The value
of "N" is applied to multiplying circuits 82 and 90 so that their
transmission coefficients vary in unison in response to changes in
the value of "N". It will now be evident that if, for example,
seven frames have passed since motion was last detected (i.e., N =
7), one-eighth of the present video signal will be added to
seven-eighths of the stored video signal and the sum applied to the
delay device 86. Similarly, if only two frames have passed since
motion was last detected, one-third of the present video signal
will be applied to the summer for addition to two-thirds of the
stored video signal.
The value of N is determined by comparing the incoming video signal
on line 80 with the delayed or stored video from chroma inverter 88
in a comparator circuit 94, the function of which is to detect
motion, on a picture element-by-element basis, between the incoming
and stored video signals. the comparator 94 may be of the form
shown in the left-hand portion of the system of FIG. 2, namely,
consisting of a differential amplifier to the two input terminals
of which are respectively applied corresponding bits of the encoded
incoming signal and the encoded stored video, and an integration
comparator for comparing the difference number at the output of the
differential amplifier with a preselected integration or reference
number, the value of which may be adjusted by the "threshold level
adjust" schematically illustrated at 96 in FIG. 5. If the
comparison produces a difference which does not exceed the preset
threshold level a signal is produced at a first output line 98
signifying "no change" in that incoming frame just compared, which
signal is coupled through a gating circuit 100 actuated by timing
pulses derived from the incoming video signal, to the tag store 92
to be counted. So long as the detected motion between successive
incoming frames and the stored video is less than the preset
threshold amount, such "no change" signals are applied to the tag
store so as to increase the value of N, which, as has been noted
earlier, equals the number of frames of incoming video since motion
in excess of the threshold level was detected. If, however,
comparison of the incoming and stored video results in a difference
in excess of the threshold level, indicative of a relatively large
amount of motion in a given picture element, the comparator
produces a signal on output line 102 which when applied to the tag
store 92 resets the register to thereby, in effect, cause the value
of N to go to zero. When this occurs, the value of the N/N+1
relationship goes to zero, and the relationship 1/N+1 becomes
unity; that is, none of the stored signal is applied to the summer
84 and only the incoming video signal is applied to the delay
device 86 to become the output signal. Thus, is detected motion is
less than a preselected threshold amount, the level of which can be
adjusted, the system continues to average the preceding and
incoming frames and to apply an increasingly larger fraction of the
stored video signal to the summer, and when motion exceeds the
preset threshold, passes only the present video signal to the
output line. It is to be noted that in the period immediately
following detected motion in excess of the preselected threshold,
if there is not excessive motion in the next succeeding incoming
frame the value of N goes to one, with the consequence that
one-half of the amplitude of that frame is summed with the next
succeeding incoming frame thereby to cause the noise reduction
action to occur. very quickly. This, the system does not "freeze"
in noise that had been contained in immediately preceding
frames.
The configuration of FIG. 5 functions as an "ideal" integrator in
that it adds picture elements of the present or incoming frame to
the same picture elements of N previous frames all with equal
weight, thereby to obtain the highest possible signal-to-noise
improvement, and this with a single delay device. It is essentially
a form of recursive filter that achieves ideal signal-to-noise
improvement yet requires only one delay device of one frame storage
capacity together with a suitable register to count the number of
samples making up the average.
It will be seen that although the configurations of FIGS. 1 and 5
utilize different averaging techniques, they have the common
function of controlling the relative fractional amplitude portions
of the incoming and stored video signals to be summed and averaged
as a function of motion between the incoming and the stored video
signals.
While the invention has been described with reference to two
specific embodiments, along with suggested modifications to adapt
them to other applications, it is intended that such modifications,
and others that will now be apparent to one skilled in the art, be
encompassed by the following claims.
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